The present invention relates to a process for the preparation of N,N'-disubstituted guanidines from the corresponding N,N'-disubstituted thioureas and ammonia.
Substituted guanidines find wide application especially as vulcanization accelerators; the most important representatives being N,N'-diphenylguanidine (I; DPG) and di-o-tolylguanidine (II; DOTG); ##STR1##
Both products are synthesized on an industrial scale by reacting aniline with cyanogen chloride. This proceeds via the following stages: ##STR2##
After the last stage of the reaction, removal of aniline by means of steam, purification with activated charcoal and sodium sulfite, filtration and, finally, precipitation with sodium hydroxide solution are necessary. This process is described in ULLMANNS ENCYCLOPEDIA, 4th edition, Vol. 12, page 416.
The major disadvantages of this process are the above-mentioned processing and purification steps and the use of the highly poisonous cyanogen chloride. In addition, the latter compound must first be synthesized by a preceeding reaction from chlorine and sodium cyanide (a more recent method of manufacturing cyanogen chloride starts with chlorine and hydrogen cyanide; Unexamined West German Application DE-OS 21 54 721).
The synthesis of N,N'-disubstituted guanidines by reaction of the corresponding N,N'-disubstituted thioureas with ammonia is also known in the art. For example, West German Patents DE-PS 418,100, 481,994, 630,966, and 632,130 describe processes in which the thioureas and ammonia are reacted with lead monoxide, zinc oxide, or lead salts, e.g., according to the following equation: ##STR3##
However, these reactions proceed very slowly and are incomplete. In addition, reaction mixtures of guanidine, the starting thiourea, lead monoxide and lead sulfide, as well as other by-products are formed, which are very difficult to separate. A complete reaction can be achieved only if the lead monoxide is used in great excess.
The Hungarian Teljes 17.165 also discloses a process for synthesizing N,N'-diphenylguanidine by reacting N,N'-diphenylthiourea and ammonia in an organic solvent with lead or zinc oxide, hydroxide, oxyhydroxide, etc., and 1 to 10 weight % sulfur catalyst.
Soviet Union patent 168,683 discloses a process for synthesizing diphenylguanidine by reacting diphenyl-thiourea and ammonia in the presence of an oxidation agent, such as, for example, sodium or potassium bichromate or manganese peroxide. Thus, according to these known processes, heavy metal oxygen compounds are used which participate stoichiometrically in the chemical reaction. By-products of the reaction include heavy metal oxides, sulfides and sulphur oxidation products, so that here, too, a complicated process for isolating the product must be carried out. Yields of diphenylguanidine in the above process are less than 90%.